It is not uncommon for operators of battery-powered portable communications devices, such as wireless telephones, personal data assistants, wireless pagers, and portable computers, to carry one or more spare batteries to extend the operational time of the device. External-type chargers are popular because they allow the operator to continue using the communications device while the other battery charges. However, typically portable chargers only allow a single battery to be charged at a time. Such “single-capacity” chargers have the obvious disadvantage of requiring lengthy recharge periods, particularly where the operator has multiple spare batteries. Therefore, attempts have been made to provide a battery charger that allows the operator to charge more than one battery at a time.
For instance, Brake (U.S. Pat. No. 5,780,991) describes a battery charger having multiple charging stations. If a battery is inserted in each charging station, each battery is charged in sequence until the voltage of the battery under charge reaches the rated battery voltage.
Johnson (U.S. Pat. No. 5,028,859) describes a battery charger having a pair of charge pockets, one of which is assigned charging priority over the other. If a battery is inserted in each pocket, the battery in the priority pocket is fast charged with a fixed high current until a predetermined voltage is reached. Thereafter, the battery is trickle charged with a fixed low current, and the battery in the non-priority pocket is fast charged with a fixed high current.
Kim (US 2004/0108835) describes a microcomputer-based battery charger having a current source and pair of switching devices for alternately charging a pair of batteries. During a first charging interval, the current source charges the first battery via the first switching device until the voltage of the first battery reaches a predetermined level (e.g. 70% of full charge). During the next charging interval, the current source charges the second battery via the second switching device until the voltage of the second battery reaches a predetermined level. During subsequent charging intervals, the batteries are again alternately charged to successively higher voltage levels.
None of these solutions make optimum use of the available charge current. As a result, charge times are unnecessarily long.